Fail safe vacuum type circuit interrupter and associated load current tap changer for electric induction apparatus

ABSTRACT

A load current tap changer for electric power transformers includes a vacuum type arcing-duty current interrupter immersed in dielectric fluid and biased to closed position by differential pressure. Overtravel limit switch means is actuated to shunt the vacuum arc gap upon excessive contact separation due to loss of vacuum and contact bias force. Limiting contact overtravel also disables the tap selector driving means.

1111 3,720,867 1 1March 13, 1973 United States Patent Rathbun [54] FAILSAFE VACUUM TYPE CIRCUIT 3,472,981 10/1969 McCarty et a1................1..200 144 B 3,582,587 .200 144 B 3,626,123

6/1971 Barkan.............................

INTERRUPTER AND ASSOCIATED 12/1971 Pflanz et al. .............200/144 BLOAD CURRENT TAP CHANGER FOR ELECTRIC INDUCTION APPARATUS PrimaryExaminer-Gerald Goldberg Attorney-Francis X. Doyle et a1.

[75] Inventor: William H. Rathbun, Pittsfield,

Mass.

General Electric Company Feb. 4, 1972 Filed: A load current tap changerfor electric power trans- [211 App! NOJ 223,631 formers includes avacuum type arcing-duty current interrupter immersed in dielectric fluidand biased to closed position by differential pressure. Overtravel limitswitch means is actuated to shunt the vacuum arc gap upon excessivecontact separation due to loss of vacuum and contact bias force.Limiting contact overtravel also disables the tap selector drivingmeans.

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References Cited 12 Claims, 3 Drawing Figures 3,404,247 Glassanos.........................2o0/144 B PAIENTED MAR 1 3 I975 SHEET 1 BF 2FAIL SAFE VACUUM TYPE CKlRCUllT HNTIERRUPTER AND ASSOCllATlElD ILQADCURRENT TAP CHANGER FOR ELECTRIC HNDUCTllON APPARATUS My inventionrelates to protective means for vacuum type circuit interrupting devicesand for load current tap changing apparatus including such devices, and

' more particularly to fail-safe means for preventing damage to suchapparatus and devices in the event of significant loss of vacuum in aninterrupting device immersed in a body of liquid. Published prior artpresently known to applicant and concerning apparatus of this generaltype is illustrated in the following U.S. Pat. Nos.: 3,265,840 toLuehring; 3,404,247 to Glassanos; and 3,472,981 to McCarty et al.

Vacuum type electric current interrupting devices of high interruptingcapacity are useful in electric switches and circuit breakers of alltypes, and are presently available in voltage and current ratingscapable of interrupting power flow at voltages and frequencies commonlyused in commercial and industrial distribution and transmission systems.in most applications the vacuum interrupting device is likely to belocated in an ambient atmosphere of air, so that even a large leak inthe evacuated envelope can do no more than fill the envelope with air orgas at ambient pressure. Under such conditions the device may fail tointerrupt current, but the contained gas or air is not likely to buildup sufficient pressure to fracture the envelope. in certain specialapplications it is desirable to immerse a vacuum interrupting device ina surrounding ambient liquid. In such locations it is possible that aleak of appreciable size and time duration will admit to the switchenvelope a pool of liquid sufficient, if heated excessively by an arc,to develop a bursting pressure.

One special application where it is desirable to immerse a vacuuminterrupter in a surrounding ambient liquid is in load tap changingapparatus for power transformers or other electric induction apparatus.Such transformers are usually immersed in a dielectric liquid such asoil or askeril, and the tap changing equipment is also liquid immersed.However, the tap changer ordinarily includes an arc current interruptingdevice which has been located in a separate liquid filled compartment toavoid contamination of the main body of transformer oil as a result ofarching at the switch. To accommodate the mechanical interconnection ofthe arching switch with the transfer contacts and other switches in atap changing apparatus, it has been customary to locate the entire tapchanging apparatus in the separate liquid filled compartment. Thisrequires all the tap leads from the transformer winding to be broughtout from the main body of the transformer to the tap changingcompartment. in high voltage apparatus this is an expensive procedure inboth labor and material costs.

It would be desirable, if possible, to locate all the tap changingapparatus for high voltage power transformers, reactors and the likedirectly within the main enclosing tank and immersed in the main body oftrans former oil or other dielectric liquid. This economical arrangementdoes become possible when the are interrupting switch is of the vacuumtype. With the entire tap changing apparatus so located, the tap leadsneed not be brought out of the tank, and no separate tap changingcompartment need be provided. Even if a separate compartment isotherwise desirable, a vacuum interrupting device is advantageous in aload tap changer in order to avoid contamination of the oil in thatcompartment and the consequent frequent maintenance. On the other hand avacuum interrupting device so located in the dielectric liquid of anassociated electrical apparatus should not be permitted to develop leakssufficiently large to admit liquid to the switch envelope. Even in theabsence of further damage, a fractured interrupter would so contaminatethe dielectric liquid that expensive and time consuming maintenance workwould be required.

Accordingly it is a principal object of my invention toprovide improvedleak responsive protective means for vacuum type electric circuitinterrupters and associated load current tap changing apparatus.

it is a more particular object of my invention to provide means forpreventing long continuance of arcing in a vacuum interrupting devicesubjected to abnormal increase in internal pressure.

it is still a further object of my invention to provide a vacuum typeelectric circuit interrupter provided internally with means responsiveto abnormal increase of internal pressure to short circuit the contactsand terminate arcing therebetween.

in carrying out my invention in one preferred embodiment, l utilize tapchanging apparatus of the type including two movable tap selectorswitches connected in parallel circuit relation to a common lineterminal and adapted to be moved in sequential step by step manner fromone tap position to an adjacent tap position. The parallel branchcircuit connections of the selector switches preferably include currentlimiting reactors and are adapted to be interrupted sequentiallyprior-to stepping movement of each associated tap selector switch. Forsuch interruption l utilize a vacuum type circuit interrupter, eitherone in each branch circuit or a single interrupter connected to'beswitched into the branch circuits sequentially during the interruptinginterval of each circuit.

A vacuum interrupting device of the evacuated envelope type is usually anormally closed contact device by reason of the fact that the movablecontact rod is sealed to the enclosing envelope by means of a flexibleor deformable wall portion of the envelope (such as a metallic bellows),and external pressure on such wall portion biases the movable contact toengaging position. Thus actuating force applied to the movable contactrod to move it to disengaging position must act against the closing biasof the bellows or other flexible seal. If the actuating mechanism forthe movable contact permits overtravel of the movable contact beyond itsnormal open circuit position abnormal overtravel will result from somepredetermined diminution or reversal of differential pressure onopposite sides of the flexible wall seal.

In such a vacuum type load current tap changing apparatus l utilizeovertravel, or abnormal opening movement, of the movable vacuuminterrupter contact resulting from abnormally high internal pressure inthe evacuated envelope to effect contacting engagement of a shuntingdevice either within or outside the envelope, thereby to extinguisharcing between the contacts and prevent further build up of pressure.The abnormal pressure effective to actuate the shunting device ispreferably less than a pressure which would rupture the switch envelope.In a preferred embodiment of my invention l utilize a metallic vaporshield within the vacuum interrupter envelope as the contact shuntingdevice by so mounting the shield that when engaged by the movablecontact in abnormal overtravel it completes a shunt circuit between thecontacts. Such a shield may be normally connected electrically to one ofthe contacts or insulatingly mounted in electrostatically floatingrelationship.

In order to prevent further tap changing movement of the selectorcontacts with the vacuum interrupting device shunted and inoperable, andthus to eliminate arcing at the tap selector contacts, I also providemeans actuated by abnormal overtravel of the vacuum switch contact toblock the tap changing mechanism or disable the motor drive for themechanism, or both.

My invention will be more fully understood and its several objects andadvantages further appreciated'by referring now to the followingdetailed specifications taken in conjunction with the accompanyingdrawing wherein:

FIG. 1 is a schematic illustration of a transformer load tap changingapparatus including a vacuum type circuit interrupting device andembodying my invention; and

FIGS. 2 and 3 are fragmentary cross-sectional views of vacuuminterrupting devices illustrating other embodiments of contact shuntingdevices which may be utilized in the apparatus of FIG. 1.

Referring now to the drawing, l have illustrated at FIG. 1 a highvoltage autotransformer of the liquid immersed type including a tank orenclosure substantially filled with insulating oil or other dielectricfluid 11 and having positioned therein a winding 12. The transformerwinding 12 is grounded at its low voltage end, as by connection to thetank 10,'and at its other end it is connected to a high voltage lineterminal conductor 13 which passes out of the tank through an insulatingbushing 14. The high voltage end of the autotransformer winding isprovided with a plurality of selectable voltage taps each having aterminal contact 15. A selected one of the tap terminals is adapted tobe connected through a load tap changing apparatus 16 to a secondaryline terminal conductor 17 disposed in an insulating bushing 18. Theload tap changing apparatus designated generally by the reference number16 comprises a pair of movable tap selector contacts or fingers 20, 21,associated pairs of current limiting reactors 22, 23 and transferswitches 24, 25, a vacuum interrupting switch 26 and a motor drivencycle timer (designated generally as 28) for controlling the operationof the foregoing tap selector contacts and switches.

The tap selector circuit of the apparatus 16 is of a well known type.This circuit provides two parallel current paths, or branch circuits,between any selected tap terminal and the common secondary line terminal17, each branch circuit includes one of the tap selector contacts 20, 21in series with one of the reactors 22, 23 and one of the transferswitches 24, 25. Each transfer switch 24, 25 has a line contactconnected to the terminal 17 and a load contact connected to one of thetap selector contacts 20, 21. The arcing duty vacuum interrupting device26 is connected between the parallel branch circuits on the load sidesof the transfer switches 24, 25 so that either transfer switch whenopened is shunted by the vacuum interrupter and the other transferswitch.

The cycle timer 28 is selectably operable in either direction to effectsequential stepping movement of the tap selector fingers 20, 21progressively in either direction along the group of transformer tapterminals 15. The cycle timer is driven selectively in either directionby a motor 30 through a reversing switch 31 from a suitable source ofcontrol voltage shown as a tertiary winding 32 on the transformer 12.When energized the timer starts from a normal rest position and operatesfor a single complete cycle to effect a single stepping movement of bothtap selector contacts 20 and 21. The motor energizing circuit includes anormally open starting switch 33 which for the purpose of illustrationmay be considered to be manually operable. The cycle timer includes amotor holding and limit switch 35 connected in parallel circuit relationacross the starting switch 33 and arranged to deenergize the motor atthe end of a single cycle in either direction of operation.

The cycle timer 28 comprises a pair of earns 40, 41 arranged to open thetransfer switches 24 and 25 sequentially in either direction ofoperation, and a cam 43 arranged to open the arcing contact switch 26 attwo spaced apart intervals during each cycle. A pair of Geneva gears 44,45 driven by the cycle timer is connected to actuate the tap selectorcontacts 21 and 20 in sequential relation at appropriate times during acycle of operation in either direction.

A typical single cycle of operation of the tap changing apparatusdescribed above takes place as follows: If the starting switch 33 isclosed momentarily to energize the motor 30, the cycle timer begins torotate in a selected direction (assume clockwise) as determined by thereversing switch 31. Immediately upon initiation of operation, theholding and limit switch 35 closes to maintain motor energization for afull cycle. During this cycle of operation the cam 40 first opens thetransfer switch 24, thereby to substitute the contacts of the arcingcontact switch 26 for the switch 24 in the branch circuit through thetap selector contact 20 and reactor 22. Following opening of thetransfer switch 24, the contacts of the arcing vacuum switch 26 areopened by the cam 43, thereby to interrupt current through the tapselector contact 20. Thereafter the Geneva gear 45 moves the tapselector contact 20 to the next adjacent tap terminal 15 in the selecteddirection. Following this operation the cam 43 again closes the contactsof the arcing contact vacuum switch 26, and cam 40 closes transferswitch 24; thereafter the cam 41 opens the transfer switch 25. Afteropening of the switch 25 the cam 43 again opens the vacuum switch 26thereby to interrupt current through the tap selector contact 21 and thereactor 23. While the tap contact 21 is deenergized, the Geneva gear 44moves that tap contact to the next adjacent tap terminal 15 to which thetap contact 20 was previously moved. Thereafter the arcing contactvacuum switch 26 is again closed by the cam 43, and the transfer switch25 is reclosed by the cam 41. This completes a single cycle of operationand the limit switch 35 thereupon deenergizes the motor 30. It will beevident to those skilled in the art that operation in the reversedirection is entirely similar.

Reference will now be had more particularly to the arcing contact vacuuminterrupter 26 connected between the parallel tap selector branchcircuitsv This circuit interrupter comprises an evacuated envelope 50,illustrated as a cylinder of insulating material such as glass havingmetallic end closure caps 50a and 50b and having mounted therein a pairof separable contacts 51, 52. The vacuum switch contacts 51, 52 aremounted, respectively, on the juxtaposed ends of axially aligned fixedand movable contact rods Sin and .5211. Contact rod 51a is fixed to themetal end cap 50a and the contact rod 52a is slidably mounted in abearing 56 which is mounted upon the end cap 50b. The slidable contactrod 52a extends through the end cap 50b and is sealed to the end cap bymeans of a flexible metallic bellows 57. The bellows 57 is connected atone end to the end wallStlb of envelope 50 and at the other end to thecontact rod 52a and thus constitutes a flexible portion of the envelopewall.

The bearing 56 does not provide a hermetic seal so that the interior ofthe bellows 57 is exposed to ambient pressure outside the switchenvelope 50. In the example illustrated this is the pressure of thesurrounding liquid body Ill and is ordinarily approximately atmosphericpressure. Since this external pressure is greater than the very lowpressure within the highly evacuated envelope 50 it exerts upon theflexible bellows 57 a force tending to expand the bellows and move theswitch contact 52 to its engaged or circuit closing position as shown inthe drawing. Thus to separate the contacts 51, 52 and to open the vacuumswitch it is necessary to forceably pull the contact rod 52a outward (ordownward as shown in the drawing) and to this end the opening cam 43 onthe cycle timer 28 is illustrated as applying a downward force upon itscam follower. In the drawing I have illustrated by dotted lines adjacentthe movable contact 52 the closed, or engaged, position (C) of contact52, the normal open or disengaged position of the contact 52, and anovertravel or abnormally displaced open position (OT) of the contact 52.Abnormal displacement to position OT will be more fully describedhereinafter.

Mounted coaxially within the vacuum interrupter envelope 50 l have showna cylindrical metallic vapor shield 60 with the contacts 511, 52 andcontact rods 51a, 52a in the central arcing region of the envelope. Theshield 60 is mounted upon the insulating envelope t) and normally notelectrically connected to either of the contacts 51, 52 so that it is inelectrostatically floating relation with respect to the contactpotential. A shield mounting member 61 between the shield 60 and theenvelope 50 is designed to be frangible or otherwise deformable underforces to be described hereinafter, but normally holds the shield 60 infixed position out of engagement with the vacuum switch contacts androds in both engaged and normal disengaged positions of the contacts.

In the embodiment of my invention herein described by way ofillustration the electrically conductive me'tallic vapor shield 60 andthe frangible, or otherwise deformable, shield mounting till is utilizedto provide a conductive shunt connection across the arc gap and betweenthe contacts 51, 52 in the event the movable contact 52 is actuated toits abnormally displaced overtravel position OT. For this purpose theshield is provided at each end with in-turned end collars approachingbut not engaging the aligned contact rods Sla, 52a and positioned forengagement with conductive interference detents or projections 51b, 52b,respectively, on the contact rods.

It will now be evident to those skilled in the art that in the normalcyclic operation of tap changing apparatus described above the vacuuminterrupter contacts 51 and 52 open and close recurrently withoutengagement of any contact parts with the enclosing vapor shield 60. Inthe event, however, that an abnormal increase in pressure occurs withinthe vacuum switch envelope 50, as for example because of a leak in oneor more of the sealed joints constituting the envelope, the closing biasexerted by the bellows 57 will be partially or fully overcome dependingupon the extent of internal pressure increase. Of course even withoutany leak, and in the absence of foreign material within the evacuatedenvelope, internal pressure increases in normal circuit openingoperation as the result of arcing between the contacts and the presenceof metal vapor. Such normal increase in pressure is not sufficient, todrive the movable contact 52 beyond the normal con tact opening position0. At some predetermined high pressure, however, contact 52 will bedriven to its overtravel position OT. Preferably the abnormally highpressure effective to produce such overtravelis less than that whichwould rupture the envelope 50.

In the event of leakage in the vacuum envelope 59, foreign material suchas gas or liquid within the envelope will result in an increase ininternal pressure which, without arcing, may or may not be sufficient toreverse the closing bias on the bellows 57. If, however, such increasedinternal pressure is of sufficient magnitude that further increase dueto contact arcing does reverse the normal closing bias the movablecontact rod 52a will be driven in the opening direction beyond itsnormal open position 0 and toward the abnormally displaced overtravelposition OT. In moving to this position the contact rod projection 52bengages the lower end collar of the conductive shield 60, and if theopening pressure is sufficient the shield mounting 61 will be broken orotherwise deformed and the shield 60 moved downwardly into engagementwith the contact rod projection 51b. Thus by excessive opening movementof the contact rod 52b the metallic shield 60 is brought into shuntingrelation with the arc gap and operates as a limit switch to bridge theseparated contacts 51, 52 and terminate arcing within the envelope 50.Bythis action the vacuum switch 26 is protected against further increasein internal pressure and against possible rupturing forces.

In the event of abnormal overpressure within the vacuum switch 26 andconsequent failure of the switch to interrupt current it is desirable toprotect the tap changing apparatus by disabling the cycle timer 28 andthe stepping selector contacts 20, 21. For this purpose I have shown ablocking gear on the cycle timer shaft which is adapted to be engagedand held in fixed position by a detent 7ll actuated by the movablecontact rod 52a. It will be understood that the detent 71 engages theblocking gear 70 only when the contact rod 52a is in its abnormallydisplaced overtravel position. On the drawing at FIG. 1 I have indicatedby dotted lines the positions of detent 71 corresponding to the closed(C) open (0) and overtravel (OT) positions of the movable contact rod52a.

. tively shunt said are gap.

As an alternate, or additional, means for disabling the cycle timer 28 lhave shown also a clutch 75 between the driving motor 30 and the cycletimer shaft. By a schematic coupling connection 76 I have illustratedthe clutch 75 as being actuated to open position only when the movablecontact rod 52a is in its abnormal overtravel position.

At FIG. 2 I have illustrated another embodiment of overtravel shuntingdevice operable by a vacuum interruption contact and operable in theapparatus shown at FIG. 1. In the vacuum interrupter shown at FIG. 2 theconductive vapor shield 60 is physically mounted upon and electricallyconnected at its upper end to the fixed contact rod 51a. An in-turnedend collar at the lower end of the shield is positioned to be engaged bythe movable contact 52 only in its overtravel position, as indicated onthe drawing.

At FIG. 3 l have shown still another form of vacuum interrupter in whicha shunting switch external to the evacuated envelope 50 comprises afixed contact 80 connected electrically to the fixed vacuum contact 51and a movable contact 81 mounted on the movable contact rod 520. Asindicated the limit switch contacts 80, 81 are positioned to engage onlyin the overtravel position OT of the contact rod 52a.

While I have shown and described a particular embodiment of myinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from myinvention in its broader aspects; and I, therefore, intend herein tocoverall such changesand modifications as fall within the true spiritand scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A load current tap changer for electrical apparatus having aplurality of tap terminals and a line terminal comprising, a pair ofmovable tap selector contacts mounted for sequential stepping movementin either direction between adjacent tap terminals, a pair of parallelbranch circuits each connecting one of said tap selector contacts tosaid line terminal, each said branch circuit including a transfer switchhaving separable line and load contacts, a vacuum type circuitinterrupting device connected between the load contacts of said transferswitches and including separable arcing contacts within an evacuatedenvelope, said arcing contacts being biased into engagement bydifferential pressure within and without said envelope, means actuatingsaid transfer switches alternately to open position thereby to transfersaid arcing contacts into one said branch circuit, means actuating saidarcing contacts to normal disengaged positions separated by an arc gapthereby to interrupt current through said one branch circuit prior tostepping movement of the associated tap selector contact, said actuatingmeans and at least one said arcing contact being free to overtravelbeyond its normal disengaged positionin response to abnormal increase ofpressure within said envelope thereby to in-- crease the length of saidarc gap, and limiting contact means engageable upon predeterminedovertravel and abnormal separation of said arcing contacts to conduc- 2.A tap changer according to claim 1 wherein said limiting contact meansis mounted within said envelope and actuated to engaged position inresponse to predetermined abnormal overtravel of at least one of saidarcing contacts.

3. A tap changer according to claim 1 wherein said limiting contactmeans comprises a cylindrical metal shield mounted within said envelopeand enclosing said arc gap in coaxial radial spaced relation and acontact means actuated by at least one of said arcing contacts to engagesaid shield in response to abnormal overtravel of said one arcingcontact.

4. A tap changer according to claim 3 wherein one said arcing contact isfixed and the other movable, normally fixed deformable mounting meanselectrostatically isolated from said arcing contacts positions saidshield normally beyond engagement with either of said arcing contactsand said movable arcing contact is positioned to engage said shield uponabnormal overtravel contact movement and move said shield intoengagement with said fixed arcing contact.

5. A tap changer according to claim 3 wherein one said arcing contact isfixed and the other movable, said shield is mounted upon andelectrically connected to the fixed arcing contact, and conductive meanson the movable arcing contact engages said shield upon predeterminedabnormal contact opening movement.

6. A tap changer according to claim 1 wherein said arcing contactscomprise a pair of axially aligned conductive rods one of which is fixedto said envelope and the other is movably mounted upon a flexible wallportion of said envelope, and said limiting contact means comprises afixed contact and a movable contact actuated by said movable contactrod.

7. A tap changer according to claim 6 wherein said limiting contactmeans is positioned outside said evacuated envelope.

8. A tap changer according to claim 1 additionally including drivingmeans for actuating said tap selector contacts and means operable uponabnormal separation of said arcing contacts to disable said drivingmeans.

9. A tap changer according to claim 8 wherein said driving meansincludes alternately and intermittently rotatable gearing connected toactuate said tap selector contacts and said disabling means comprisesblocking means engageable positively to lock said gearing againstmovement.

10. A tap changer according to claim 8 wherein said driving meansincludes a rotatable electric motor and said disabling means comprises aclutch for disconnecting said motor from said tap selector contacts.

1 l. The combination with a tap changer according to claim 1 of anelectric induction apparatus including said tap terminals and said lineterminal, a common housing for said induction apparatus and said vacuuminterrupting device, and a body of dielectric fluid in said housing atleast partially immersing said induction apparatus and said vacuuminterrupting device.

12. In combination, an electric power transformer having a plurality oftap terminals and a line terminal, an enclosing housing for saidtransformer, a body of dielectric liquid in said housing substantiallyimmersing said transformer, and the tap changer of claim 1 connectedbetween said line terminal and said tap terminals with said vacuuminterrupting device immersed in said dielectric liquid.

1. A load current tap changer for electrical apparatus having aplurality of tap terminals and a line terminal comprising, a pair ofmovable tap selector contacts mounted for sequential stepping movementin either direction between adjacent tap terminals, a pair of parallelbranch circuits each connecting one of said tap selector contacts tosaid line terminal, each said branch circuit including a transfer switchhaving separable line and load contacts, a vacuum type circuitinterrupting device connected between the load contacts of said transferswitches and including separable arcing contacts within an evacuatedenvelope, said arcing contacts being biased into engagement bydifferential pressure within and without said envelope, means actuatingsaid transfer switches alternately to open position thereby to transfersaid arcing contacts into one said branch circuit, means actuating saidarcing contacts to normal disengaged positions separated by an arc gapthereby to interrupt current through said one branch circuit prior tostepping movement of the associated tap selector contact, said actuatingmeans and at least one said arcing contact being free to overtravelbeyond its normal disengaged position in response to abnormal increaseof pressure within said envelope thereby to increase the length of saidarc gap, and limiting contact means engageable upon predeterminedovertravel and abnormal separation of said arcing contacts toconductively shunt said arc gap.
 1. A load current tap changer forelectrical apparatus having a plurality of tap terminals and a lineterminal comprising, a pair of movable tap selector contacts mounted forsequential stepping movement in either direction between adjacent tapterminals, a pair of parallel branch circuits each connecting one ofsaid tap selector contacts to said line terminal, each said branchcircuit including a transfer switch having separable line and loadcontacts, a vacuum type circuit interrupting device connected betweenthe load contacts of said transfer switches and including separablearcing contacts within an evacuated envelope, said arcing contacts beingbiased into engagement by differential pressure within and without saidenvelope, means actuating said transfer switches alternately to openposition thereby to transfer said arcing contacts into one said branchcircuit, means actuating said arcing contacts to normal disengagedpositions separated by an arc gap thereby to interrupt current throughsaid one branch circuit prior to stepping movement of the associated tapselector contact, said actuating means and at least one said arcingcontact being free to overtravel beyond its normal disengaged positionin response to abnormal increase of pressure within said envelopethereby to increase the length of said arc gap, and limiting contactmeans engageable upon predetermined overtravel and abnormal separationof said arcing contacts to conductively shunt said arc gap.
 2. A tapchanger according to claim 1 wherein said limiting contact means ismounted within said envelope and actuated to engaged position inresponse to predetermined abnormal overtravel of at least one of saidarcing contacts.
 3. A tap changer according to claim 1 wherein saidlimiting contact means comprises a cylindrical metal shield mountedwithin said envelope and enclosing said arc gap in coaxial radial spacedrelation and a contact means actuated by at least one of said arcingcontacts to engage said shield in response to abnormal overtravel ofsaid one arcing contact.
 4. A tap changer according to claim 3 whereinone said arcing contact is fixed and the other movable, normally fixeddeformable mounting means electrostatically isolated from said arcingcontacts positions said shield normally beyond engagement with either ofsaid arcing contacts and said movable arcing contact is positioned toengage said shield upon abnormal overtravel contact movement and movesaid shield into engagement with said fixed arcing contact.
 5. A tapchanger according to claim 3 wherein one said arcing contact is fixedand the other movable, said shield is mounted upon and electricallyconnected to the fixed arcing contact, and conductive means on themovable arcing contact engages said shield upon predetermined abnormalcontact opening movement.
 6. A tap changer according to claim 1 whereinsaid arcing contacts comprise a pair of axially aligned conductive rodsone of which is fixed to said envelope and the other is movably mountedupon a flexible wall portion of said envelope, and said limiting contactmeans comprises a fixed contact and a movable contact actuated by saidmovable contact rod.
 7. A tap changer according to claim 6 wherein saidlimiting contact means is positioned outside said evacuated envelope. 8.A tap changer according to claim 1 additionally including driving meansfor actuating said tap selector contacts and means operable uponabnormal separation of said arcing contacts to disable said drivingmeans.
 9. A tap changer according to claim 8 wherein said driving meansincludes alternately and intermittently rotatable gearing connected toactuate said tap selector contacts and said disabling means comprisesblocking means engageable positively to lock said gearing againstmovement.
 10. A tap changer according to claim 8 wherein said drivingmeans includes a rotatable electric motor and said disabling meanscomprises a clutch for disconnecting said motor from said tap selectorcontacts.
 11. The combination with a tap changer according to claim 1 ofan electric induction apparatus including said tap terminals and saidline terminal, a common housing for said induction apparatus and saidvacuum interrupting device, and a body of dielectric fluid in saidhousing at least partially immersing said induction apparatus and saidvacuum interrupting device.